[Development] qMoveToConst helper for rvalue references to movable Qt containers?

[Elvis Stansvik]

Den lör 27 okt. 2018 kl 19:48 skrev Lars Knoll <lars.knoll at qt.io>:
>
>
>
> On 27 Oct 2018, at 19:29, André Pönitz <apoenitz at t-online.de> wrote:
>
> On Sat, Oct 27, 2018 at 04:33:30PM +0100, Sérgio Martins wrote:
>
> On Sat, Oct 20, 2018 at 1:44 PM Elvis Stansvik <elvstone at gmail.com> wrote:
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>
> Hi all (first post),
>
>
> Welcome :)
>
> In Qt 5.7+ there's qAsConst, an std::as_const implementation for those
> who are not on C++17 yet, which is convenient for iterating over Qt
> containers using range-based for loops without causing a detach.
>
> For good reasons there's no version of qAsConst that takes an rvalue
> reference, so you can't do e.g. for (auto foo :
> qAsConst(returnsQtContainer())  { ... }. Instead you must do const
> auto stuff = returnsQtContainer(); for (auto foo : stuff) { ... }.
>
>
> Should we instead just encourage people to make returnsQtContainer()
> return a const container ?
>
>
> This is actually a route we recently took in some cases in Qt Creator's
> code base.
>
>
> That might actually make sense. Calling a non const method on the returned temporary object is usually a mistake anyway. And the copy/move assignment to a variable will work with the const object.

Hm, but was Marc not right when he said

  "Making returned containers const inhibits move semantics, because
const rvalues do not bind to the mutable rvalue references that move
constructors and move assignment operators use."

on his blog?

Guess he should jump in here to defend himself :)

Elvis

>
> Cheers,
> Lars
>
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